In the automotive field, there exist several types of power transmission pulley enabling accessories to be decoupled.
A first type is constituted by freewheel pulleys that transmit power in one direction only and provide decoupling by over-running in the opposite direction so as to enable velocity differences to exist between the driving portion and the driven portion. The balls, cams, or needles that mesh are made of metal, so they are rigid and thus provide coupling that is rigid.
The inner portion is preferably in the form of a cam suitable for wedging against connection elements so as to transmit torque.
Such pulleys are described in particular in U.S. Pat. Nos. 4,725,259 and 5,676,225.
During the decoupling stages that occur in the event of the outer portion decelerating quickly, the inner portion driven by pure inertia maintains a high velocity in the total absence of contact. The velocity difference between the outer driving element and the inner driven element can be sufficiently great to lead to a phenomenon of excess speed.
FIG. 2 of U.S. Pat. No. 4,725,259 (reproduced herein as FIG. 1) shows the fluctuation of the velocity over time, the velocity of the outer portion being drawn as a continuous line while the velocity of the inner portion is drawn as a dashed line.
When the velocity of the outer portion fluctuates rapidly, the driven system is thus disconnected and then reconnected with the outer portion:                if there is no friction, the velocity of the inner element is almost constant;        if there an opposing torque, the velocity of the inner element fluctuates a little; and        the level of velocity fluctuation presented by the inner (driven) portion depends on its deceleration, and thus on its opposing torque, and this deceleration can be very large if the opposing torque is high (e.g. when an alternator is delivering electricity at its maximum rate).        
There exist freewheels in which the drive motion comes from the inner portion, for which the explanation above remains valid, but should be inverted.
A freewheel pulley presents several drawbacks:
1) In coupled mode, the balls are wedged between the outer portion and the inner portion. While coupling is being established (at P), the rigid mechanical contact generates an impact, leading to an instantaneous rise in force. Once coupled, the freewheels transmits all of the motion from the driving portion, possibly together with additional jolting.
This has the consequence of imparting dynamic forces to the transmission, whether by belt, chain, gears, or direct, and leads to associated damage (slip, fatigue, wear).
2) FIG. 1 shows alternation between two modes of operation at a high frequency. The deceleration of the driven portion depends on the opposing torque: the greater the opposing torque, the harder the deceleration, and as a result the deceleration follows the motion of the driving portion more closely.
When the opposing torque is due to the driven member (alternator, converter, rotary machine, tool, etc.) taking power, its motion follows the motion of the driving portion almost exactly. Under such circumstances, the dynamic acceleration/deceleration forces are transmitted in full (slip, fatigue, wear).
The situation can be summed up as follows: if the member delivers an opposing torque (or if the amplitude of the fluctuation is small), then the freewheel remains blocked and never switches to unclutched mode.
For a V-grooved motor vehicle belt driving an alternator on a diesel engine:                while the engine is running at slow speed, its velocity fluctuates by as much as ±20% at a frequency of 30 hertz (Hz);        the belt picks up this motion from a drive pulley (crank shaft pulley) and conveys it to the alternator;        the alternator is driven via a freewheel, and this freewheel disconnects the inertia on each deceleration, and it reconnects it on each acceleration;        if electricity is taken from the alternator, then its torque reaches 10 newton meters (Nm) to 15 Nm, which suffices to brake the over-running stage, so the freewheel remains in coupled mode and no longer disconnects the alternator; and        if the freewheel unlocks, then on the next acceleration its locking is sudden and causes the belt to slip.        
Because of its inertia, the alternator thus creates dynamic torque that increases with increasing fluctuation, thus leading successively to belt slip, abrasion, noise, and/or fatigue in the bearings.
A second type is constituted by pulleys having resilient elements, with an element that is flexible in twisting being interposed between the outer portion and the inner portion. This element is in the form of a ring, a disk, a spring, pads, etc. The stiffness of this element is dimensioned as a function of the torque to be transmitted under static and under dynamic conditions. The resilient element can deform so as to absorb variations in torque, but it cannot accommodate differential rotation between the driving portion and the driven portion.
One category of pulleys with resilient elements is the so-called “decoupler” pulley of stiffness that is selected to filter all velocity variations above certain excitation frequency.
FIG. 2 shows an example of a known resilient pulley, and FIG. 3 shows its frequency response, where fr is its resonant frequency (in revolutions per minute (rpm)) and fc is its cutoff frequency (in rpm). Amplitude is plotted in decibels (dB) up the ordinate in the form of a filter ratio, with the 0 dB level corresponding to the amplitude at the cutoff frequency fc.
The transmission pulley shown in FIG. 2 presents a pulley element having an outer outline 1 adapted to the profile of the ribs of a ribbed V-belt. The pulley element rotates about a bearing 2 and presents an extension 1′ secured via a part 7′ of outer outline 6′ of a rubber body 6 whose inner outline is secured via another part 7″ to a part 3 which provides the connection with a receiver shaft, e.g. of an alternator.
Pulleys of this type can only oscillate about a mean position, since the connection between the outside and the inside is permanent.
The ratio between the amplitudes of the fluctuation of the (outer) driving portion and the (inner) driven portion varies a function of the frequency of the fluctuation.
In association, the stiffness and the inertia to be driven constitute a resonant system. Above the cutoff frequency fc of the resilient pulley, the driven inner portion fluctuates less than the driving outer portion.
These pulleys also present several drawbacks:
1) A first drawback is due to the fact that they also present a bearing (generally based on polytetrafluoroethylene (PTFE)) which supports radial forces. This friction impedes filtering since it creates an adhesion threshold below which the pulley remains stationary (and thus rigid).
2) If the velocity fluctuation is at a frequency well below the resonant frequency fr, or if jolting is progressive, then the elastic element deforms very little and the force is transmitted. The frequency spectrum (see FIG. 3) needs to be very well known at design time, since otherwise the elastic pulley can be inoperative.
3) If the velocity fluctuation is at a frequency higher than the cutoff frequency fc or if the jolt is brief, then the elastic element deforms. This deformation occurs in both directions; the energy stored in the positive direction is restored by resilient return in the opposite direction, and so on. This leads to heating and to damage of the pulley.
4) If the velocity fluctuation is at a frequency that is very close to the resonant frequency fr (see FIG. 3), then the resilient pulley amplifies the fluctuation (filter ratio>0 dB). This produces an effect that is contrary to the desired effect. This amplification is accompanied by very high levels of deformation in both directions; the energy stored in the positive direction is restored by resilient return in the opposite direction and so on. This also leads to heating and to damage.
Consider an application to a motor vehicle in which:                The engine (having N=4 cylinders) is running at low speed (idling at Vm=900 rpm) and its velocity is fluctuating at up to ±20% at a frequency Fexc of 30 Hz.        
It should be recalled that Fexc=VmN/120.                The alternator is driven by a decoupler pulley which is supposed to perform filtering from 700 rpm (23.3 Hz).        The idling speed is Vm=900 rpm (Fexc=30 Hz), so the pulley filters properly.        
Because of this absorption, the elastic element (made of rubber) deforms by close to ±20°, heats up, hardens, and can break quickly.
On starting, the engine goes quickly from a zero velocity to an idling speed (900 rpm). It thus finds itself for a short length of time at the resonant frequency fr. At this instant, the deformation altitude of the rubber element can reach ±40° and it can break very quickly if starting is repeated several times over.                Finally, if the engine decelerates suddenly or stalls, the belt which is connected to the crank shaft stops revolving, but the alternator continues to revolve because of its inertia. The belt is thus suddenly stretched and then unstretched, snapping back to shape.        
A third type of pulley is constituted by resilient clutch pulleys using springs.
In a resilient clutch pulley, e.g. as described in U.S. Pat. No. 6,083,130 (represented by its FIG. 2 reproduced herein as FIG. 4), a primary torsion spring 88 is interposed between the outer portion 120 and the inner portion 52, with the spring providing a clutch function by expanding radially. The clutch is subsequently connected to the resilient element which may be a second torsion spring 85-86.
When the primary spring contracts, there is excess pressure in the connection between the driving portion and the driven portion. Nevertheless, there is a friction threshold which maintains the connection, thus leading to a slip phenomenon.
In the direction in which the primary spring expands, that leads to a strong connection, and the resilient element (in this case a spring) takes over to transmit the forces.
Above a certain angle of twist, the resilient element can no longer deform (turns touching). The spring must therefore be very stretched out which increases the axial length of the pulley and thus its bulk.
The ball bearings 118 operate with a large offset which reduces their lifetime. In order to take up a fraction of the radial forces, a PTFE bearing 110-112 is interposed at the end opposite from the ball bearing. This PTFE bearing generates friction which is added to that of the clutch and reduces efficiency in unlocked mode.
A variant of this third type of pulley is described in U.S. Pat. No. 6,394,248, in particular in FIGS. 2 and 3 thereof which are reproduced herein as FIGS. 5a and 5b. The principle of the expanding spring 22 is conserved. However, the resilient element is a succession of compression springs that are interposed between fins 27 and 47.
The amplitude of deformation is necessarily limited because the springs cannot be compressed fully.
Furthermore, the elements 17 and 26 ought to have low-friction guidance, but they do not, so there is therefore a high risk of seizing.
In that construction, the radial force of the belt is supported by the ball bearing 50 which has a large offset.
The above analysis shows that:                freewheel devices are too rigid once locked, and their effectiveness depends on the opposing torque;        resilient pulleys do not enable the driven member to continue its motion in the event of deceleration; the internal friction is harmful to filtering; and        pulleys with a resilient clutch using a spring are complex and have the same friction problem; they are also bulky.        